As is well known, the transmission path used for the transmission of signals in telecommunications connections causes interference to telecommunications. This occurs regardless of the physical form of the transmission path, whether it is a radio link, optical fibre or copper cable. Especially in radio communications, there are often situations, in which the quality of the transmission path varies from one connection to another and even during a connection. Fading of the radio path is one typical phenomenon that causes changes in the transmission channel. Other simultaneous connections can also cause interference that may change with time and location. In a typical radio communications environment, signals between a transmitter and receiver propagate along several routes. This multipath propagation occurs mainly due to the fact that the signal reflects from the surrounding surfaces. The signals that have propagated along different routes arrive at the receiver at different times due to their different propagation delays. Different methods have been developed to compensate for the fading caused by multipath propagation.
One of the most effective ways to compensate for the fading on the radio path is controlling the transmission power of the transmitter. If the properties of the radio path are known, the power of the transmitter can be controlled in such a manner that the effect of the fading can be eliminated. In practice, such a solution is, however, very difficult to implement, because firstly, the transmitter should know the quality of the channel, and transmitting this information to the transmitter in real time is difficult. Secondly, transmission power restrictions set on transmitters and the dynamic range of transmitters set their own limitations. In addition, power control in itself may lead to ineffective transmission by increasing power to a high level in fading dips. A second solution to the problem is using diversity in the transmitter. Time diversity employs interleaving and coding to provide temporal diversity to the transmitted signal. However, this has the drawback that it causes delays in the transmission especially when the channel is a slow-fading one. In frequency diversity, the signal is transmitted on several frequencies at the same time. This is, however, an ineffective method when the coherence bandwidth of the channel is wide.
In transmission antenna diversity, the same signal or different parts of the same signal are transmitted to the receiver by using two or more antennas. The signal components that multipath-propagated through different channels will then probably not suffer from interference from a simultaneous fading. Publication WO 99/14871 discloses a diversity method especially suited for two antennas, in which the symbols to be transmitted that are made up of bits are coded in blocks of given lengths and in which each block is coded to contain a given number of channel symbols according to formula (1).
                              C          Ala                →                  (                                                                      z                  1                                                                              z                  2                                                                                                      -                                      z                    2                    *                                                                                                z                  1                  *                                                              )                                    (        1        )            
In the formula, the horizontal lines of the matrix show transmission time instants so that the upper horizontal line shows information transmitted at time instant t and the lower horizontal line shows information transmitted at time instant t+T, wherein T refers to a symbol cycle. The vertical lines of the matrix in turn show antennas so that the first vertical line shows information transmitted through antenna 1 and the second vertical line shows information transmitted through antenna 2. However, a block code of complex modulation as shown in formula (1) only exists for a maximum of two antennas. A maximum data transmission rate, i.e. coding rate, transmitted to more than two antennas is calculated according to formula (2), where N is the number of transmission antennas and the square brackets indicate the smallest integer that is greater than or equal to the expression in brackets. It should be noted that herein the coding rate refers to a symbol-coding rate, i.e. the number of symbols transmitted during a symbol cycle T. For a third and fourth antenna, the maximum coding rate for an orthogonal code is ¾.
                    Rate        =                                            [                                                log                  2                                ⁢                N                            ]                        +            1                                2                          [                                                log                  2                                ⁢                N                            ]                                                          (        2        )            
Publication Tirkkonen, Boariu, Hottinen, IEEE 6th Symposium on Spread-Spectrum Tech. & Appli., NJIT, New Jersey, USA, September 2000, discloses some solutions of a full coding rate 1. The publication describes a coding method, in which a non-orthogonal block code according to formula (3) having four antennas is formed by utilizing an orthogonal Alamout matrix according to formula (1).
                              (                                    z              1                        ,                          z              2                        ,                          z              3                        ,                          z              4                                )                →                  (                                                                      z                  1                                                                              z                  2                                                                              z                  3                                                                              z                  4                                                                                                      -                                      z                    2                    *                                                                                                z                  1                  *                                                                              -                                      z                    4                    *                                                                                                z                  3                  *                                                                                                      z                  3                                                                              z                  4                                                                              z                  1                                                                              z                  2                                                                                                      -                                      z                    4                    *                                                                                                z                  3                  *                                                                              -                                      z                    2                    *                                                                                                z                  1                  *                                                              )                                    (        3        )            
The matrix is of form ABBA according to presentation method (4), in which the A matrix follows the Alamout matrix for symbols z1 and z2, whereas B follows the Alamout matrix for symbols z3 and z4.
                    (                                            A                                      B                                                          B                                      A                                      )                            (        4        )            
Known non-orthogonal block codes do, however, contain a significant drawback. Said block codes do not have full diversity, i.e. the number of independently decoded channels is smaller than the number of antennas, whereby transmission capacity provided by the antennas is lost. The diversity of the block code is the smallest number of nonzero eigenvalues.Hce=DceHDce,  (5)where Dce is defined by formula (6).Dce=Cc−Ce  (6)
In the above, Cc is a transmitted code matrix and Ce a detected defective code matrix. In the case of a channel symbol pair, in which the same error Δ occurs with symbols z1 and z3 and no error occurs with symbols z2 and z4, a matrix according to formula (7) is obtained as the difference matrix Dce.
                              D          ce                =                  (                                                    Δ                                            0                                            Δ                                            0                                                                    0                                                              Δ                  *                                                            0                                                              Δ                  *                                                                                    Δ                                            0                                            Δ                                            0                                                                    0                                                              Δ                  *                                                            0                                                              Δ                  *                                                              )                                    (        7        )            
The matrix according to formula (7) is singular, i.e. the matrix does not have an inverse matrix. The matrix has only two nonzero eigenvalues, 2Δ and 2Δ*. Thus, the diversity degree of the ABBA block code according to formula (3) is only 2. Low diversity begins to show in the coding performance as a decreasing bit error rate when the bit energy to interference level ratio exceeds 5 dB.